Bigger Isn’t Always Better for Molecular Light Funnels

ROCHESTER, N.Y. -- Researchers at the University of Rochester have determined the ideal size for a light-harvesting molecule that one day may improve the light gathering of everything from photoelectric cells to genetically engineered plants. Led by physics professor Yonathan Shapir and chemistry professor Shaul Mukamel, the team has learned that after a certain size, the molecule phenylacetylene begins to lose its efficiency at channeling light to its center.

Over the past decade, scientists have experimented with artificial molecules that funnel light. Until recently, many thought that the larger the molecule, the more light it could collect. The researchers determined that this is not the case and have devised a method to determine the optimum size of such molecules.

Researchers have determined the optimum size for a molecule that channels light to its center.
To test how size affects efficiency, the researchers developed computer models to gauge how long it takes the energy of a photon to reach the core, where it causes a desired chemical reaction. "These calculations dealt with two particular aspects of the dendrimers: the nonlinear potential and the inherent randomness due to their interactions with the solvent," Mukamel said.

Useful for thermal sensors

Because dendrimers -- molecules that display a fractal branching structure -- have branches of different lengths, the excitation energy moves in a nonlinear fashion toward the center. In addition, excitations take random paths to the center and can get lost on the structure in the process.

The researchers first studied the effects separately; then they combined them. "To include both the nonlinearity and the randomness, we had to run repeated computer simulations to mimic the many different ways the randomness can take," Shapir said. They found that if the molecule is too large, either effect increases the time for the excitations to reach the center. "Luckily, the optimal sizes associated with these two different effects were found to be identical," he said, so optimizing phenylacetylene's size for one factor will not adversely affect its performance by the other.

The researchers classified the molecule by the number of times it branched, which is known as its generation. According to Mukamel, their calculations show that for phenylacetylene the optimal size is nine generations.

Because the work is in its early stages, Mukamel said, any applications are only possibilities. The model has potential in a variety of fields, tailoring dendrimers for use as drug delivery systems in medicine, as thermal sensors and in photovoltaics.

The research team published its findings in the July 10 issue of Physical Review Letters.